Effect of Chain-ends on the Mixed Polymer Brush Morphology

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PART 1: NON-TECHNICAL SUMMARYPolymer brushes are long stringy molecules that are chemically anchored to a surface to form a dense, thin coating that alters the surface properties. While the chemical anchoring to the surface provides the required mechanical strength to the coating, the chemistry and structure of the polymer enables communication with the environment. Hence, these coatings have broad relevance in solving problems in materials science and in human health. They can act as sensors for rapid detection of biomarkers in biofluids when anchored to electronically active surfaces, direct cell growth for regenerative medicine, resist deposition of micro-organisms on surfaces, control flow of charge in batteries, control friction at relevant interfaces., etc. This research program will develop new synthetic methodologies to make these polymer brushes with well defined structure and chemistry. The methodology developed is high yielding, simple and reproducible on a wide range of surfaces, hence can be adapted by the coating industry. This program will develop characterization methods to aid in the molecular level understanding of the resulting structure. The research program will serve to inspire and involve undergraduate students in polymer science and engineering research, by working with graduate student mentors, through the REU program. Educational modules will also be developed for dissemination through UW-Madison’s institute for Chemical Education, a national center dedicated to STEM education through outreach and education research. PART 2: TECHNICAL SUMMARYThe research program will develop a highly versatile approach to grow mixed A/B polymer brushes from a substrate using a cross-linkable copolymer coating, which contains a polymerizable initiator (inimer) as a comonomer. Mixed A/B polymer brushes consist of chemically distinct A and B chains, randomly grafted to the surface. They show changing density, height and composition profile as a function of solvent environment. These polymer brushes will have high chain grafting density, uniform distribution of A and B graft points, controlled molecular weights, and excellent control over composition. Theory predicts the lowering of critical point for phase separation in mixed brushes compared to block copolymers, and the formation of nanoscale morphology. However, experimental studies have fallen short of validating these predictions. The gap between theory and experiments is attributable to lack of: a) experimental methods for uniform growth of high grafting density mixed brushes with controllable grafting ratios, and predictable chain lengths, (b) accounting for chain end-groups in theory and experiments, and (c) characterization methods to study the morphology. In this project the goal is to understand the role of chain-end density, size, distribution, and type on the A/B mixed brush morphology. This is a largely experimentally and theoretically unexplored area in polymer brushes. Key aspects of this research will be to develop synthetic strategies to append select chain end groups, quantification of the extent of functionalization and chain-end distribution by a combination of X-ray photoelectron spectroscopy and Resonant Soft X-ray scattering (RSoXS), and studying the effect of chain-end functionalization on lateral phase segregation by a combination of scattering and advanced microscopy techniques. The morphological insight developed through these experiments will contribute towards the broader understanding and application of polymer brushes..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.